1. Field of the Invention
This invention relates to a dry desulfurization and denitrification process for removing SOx and NOx in effective manner from various combustion waste gases.
2. Description of the Prior Art
As a method of removing SOx and NOx from waste gases such as combustion waste gases of various fuels, burning-up waste gases of dusts, etc., it has been proposed to combine various desulfurization treatments and denitrification treatments. For example, Japanese Patent Laid-Open Publication No. 132628/1985 has proposed a method comprising removing SOx in a waste gas to 400 ppm by the use of adsorption coke, as a pre-treatment, mixing the waste gas with ammonia and treating NOx with a catalyst such as cokes containing metal oxides each having a small adsorption capacity of SO.sub.2, as an after-treatment. This technique is aiming at solving a problem on the initial contamination of activated carbon with ammonium sulfate in a denitrification bed in a method comprising providing two activated carbon adsorption beds, adding ammonia to each of these beds, carrying out predominantly desulfurization in the first bed and carrying out predominantly denitrification in the second bed.
Japanese Patent Laid-Open Publication No. 254825/1987 has proposed, as a method for treating a boiler waste gas, to carry out desulfurization and denitrification in order, followed by dust removal. In this method, the wet desulfurization is combined with the commonly used denitrification apparatus and ammonium sulfate formed by the reaction of SO.sub.3 resulting from the denitrification treatment with ammonia is collected as a dust.
Japanese Patent Laid-Open Publication No. 166214/1992 has proposed an apparatus for the desulfurization and denitrification of a waste gas from a sintering machine, ordinarily, at a temperature of 80.degree. to 130.degree. C., comprising a means for desulfurization by dry process using a carbonaceous adsorbent, a means for heating the waste gas as an after-treatment and a means for denitrification of a catalytic type, in which the denitrification is carried out at 350.degree. to 400.degree. C. In this technique, the desulfurization means by dry process is combined, because a wet process desulfurization means, used as a pre-treatment in the prior art, cannot sufficiently remove SO.sub.3, Na, K, Cl, F, etc. which lower the function of the denitrification catalyst in an after-treatment, and the temperature of the waste gas is largely lowered.
According to these methods, after the desulfurization, the denitrification is carried out by adding ammonia to a waste gas and passing it through a carbonaceous adsorbent bed and various metallic catalyst beds. The denitrification treatment using a carbonaceous adsorbent bed is carried out under conditions of a temperature range of 100.degree. to 200.degree. C. and a space velocity of at most 1000 hr.sup.-1, but this method raises a problem that the installation is large-sized.
The denitrification treatment using a metallic catalyst is ordinarily carried out by a method comprising adding ammonia to a gas containing NOx and then contacting the gas with the catalyst at a high temperature, i.e. 200.degree. to 500.degree. C., preferably 250.degree. to 400.degree. C. to decompose NOx. This method generally gives a high decomposing capacity of NOx, but, since the catalyst used herein tends to be more deteriorated with catalyst-poisoning materials such as sulfur compounds or halogen compounds and if mercury is not sufficiently removed, it adheres to the catalyst and covers the catalyst to lower the catalytic activity, it is required to sufficiently remove these catalyst-poisoning materials contained in a waste gas before being introduced in a catalyst bed.
As a pretreatment in the denitrification treatment using a metallic catalyst, removal of SOx contained in a large amount in a waste gas has mainly been taken into consideration up to the present time. In the ordinary waste gas treatment, accordingly, a waste gas is introduced into a desulfurization treatment apparatus by wet process, having achieved the most actual satisfactory results as a desulfurization means, to remove the most part of SOx in the waste gas and then fed to a denitrification apparatus.
In such a desulfurization method by wet process, SOx can be removed, but mercury or halogen compounds such as dioxin, except hydrogen halides, are hard to be completely removed, causing a shortened life of a catalyst in a subsequent denitrification apparatus. In the wet process treatment, furthermore, the temperature of a waste gas to be treated is lowered, so even when a boiler combustion waste gas at a high temperature, for example, 250.degree. to 400.degree. C. is treated, it is required to heat again the waste gas to raise the temperature thereof before introducing into the denitrification apparatus, thus resulting in a problem that the energy loss is large.
As the regulation of environment is getting severer, in addition, it has further been required for the denitrification catalyst to increase the activity thereof and to suppress formation of byproducts and various catalysts have been developed as described hereinafter. However, these catalysts have excellent properties, but they are subject to influences by catalyst-poisoning materials and respectively need a combination with an effective pretreatment.
On the other hand, various studies have been made as to the denitrification techniques and thus a number of catalysts having denitrification activity have been developed. These catalysts consist predominantly of metals such as platinum group metals, iron group metals, vanadium, chromium, cobalt, nickel and the like or oxides thereof and are ordinarily used at a high temperature, e.g. 250.degree. to 400.degree. C. Thus, a waste gas should be heated again to raise the temperature thereof before introducing it into a denitrification apparatus, resulting in a problem that the energy loss is large.
Depending upon the catalysts, nitrous oxide (N.sub.2 O), which is considered to contribute to warming up the earth and breakage of the ozone layer, is formed during decomposition step of NOx.
Regarding the treatment temperature, an denitrification catalyst active in a relatively low temperature range has been developed and for example, Japanese Patent Publication No. 2912/1979 discloses a process for the decomposition of nitrogen oxides, comprising bringing nitrogen oxides into contact with a catalyst comprising vanadium supported on titanium oxide of anatase type in the presence of ammonia at a temperature of 150.degree. to 650 .degree. C. The catalyst used in this method exhibits activity at a temperature of at least 150 .degree. C., as described in this publication, but the specific surface area thereof is so small, i.e. at most 50 m.sup.2 /g and the denitrification activity is markedly lowered at a reaction temperature of at most 170.degree. C. to lower the decomposition capacity of NOx.
That is, even these catalysts of TiO.sub.2 -V.sub.2 O.sub.5 type practically need a temperature of 200.degree. to 500.degree. C. and when using these catalysts, furthermore, there arises a problem that N.sub.2 O is by produced in a proportion of 5 to 10% based on NOx, thus resulting in possibility of the secondary pollution.
Furthermore, in spite of that these catalysts are more subject to influences by catalyst-poisoning materials, a useful desulfurization and denitrification process at a temperature of 100.degree. to 200.degree. C. in combination with a suitable pretreatment method has not been developed yet.
In the catalysts of TiO.sub.2 -V.sub.2 O.sub.5 type, a catalyst having a relatively high specific surface area and specified pore ratio has been proposed. In Japanese Patent Laid-Open Publication No. 86845/1990, for example, a catalyst for denitrification is disclosed in which the pore distribution of the anatase-type titania used as a support is controlled in such a manner that the porosity of pores (pores of at most 60 nm in diameter) is in the range of 0.05 to 0.5 cc/cc, the porosity of macropores (pores of at least 60 nm in diameter) is in the range of 0.05 to 0.5 cc/cc and the total porosity is at most 0.80 cc/cc to raise the denitrification property. Furthermore, Japanese Patent Laid-Open Publication No. 130140/1985 discloses a catalyst for the denitrification of a waste gas comprising titanium oxide, tungsten oxide and vanadium oxide as catalytic active components and having a specific surface area of 80 to 200 m.sup.2 /g, pore volume of 0.1 to 0.5 cc/g and micropores of at most 10 nm in diameter and macropores of 10 to 104 nm in diameter.
In these catalysts, however, the contacting temperature is practically adjusted to at least 200.degree. C. so as to attain a high denitrification property and if the contacting temperature is lower than this range, the catalytic effect is insufficient, for example, as shown by low denitrification property, byproducts of N.sub.2 O, etc.
In Japanese Patent Laid-Open Publication No. 200741/1992, there is proposed a catalyst comprising vanadium supported on titanium oxide of anatase type, having a specific surface area of about 150 m.sup.2 /g. This catalyst aims at adsorbing and removing nitrogen oxides, for example, contained in a ventilation gas of a tunnel with a relatively low concentration, e.g. about 5 ppm through contact with the gas, which is conceptionally different from catalysts used for catalytic decomposition. This catalyst is considered to be given a high specific surface area for the purpose of improving its capacity because of being used for the adsorption and removal of nitrogen oxide.
As the regulation of environment is getting severer, denitrification catalysts having various properties have been proposed, but for the purpose of making up an excellent desulfurization and denitrification process by the use of these catalysts, it is necessary to provide a combination with a suitable desulfurization means and conditions regarding every catalyst.